A diagenetic study of the Woodford Shale in the southeastern Anadarko Basin, Oklahoma, USA: Evidence for hydrothermal alteration in mineralized fractures

The objective of this study is to test if external fluids (e.g., hydrothermal) altered the Woodford Shale in the southeast Anadarko Basin and how the diagenesis caused by such fluids, particularly in mineralized fractures, has affected the reservoir quality and mechanical behavior of the Woodford Shale. Two Woodford Shale cores from the Anadarko Basin were sampled to identify diagenetic events, interpret their origin, and determine the diagenetic history of the shale. Thin sections for both cores were analyzed using reflected and transmitted light, cathodoluminescence (CL), and scanning electron microscopy to identify minerals and cross-cutting and textural relationships. X-ray computed tomography was conducted to further characterize fracture networks seen on the petrographic scale. Early diagenesis is dominated by events in the matrix and allochems, with later diagenesis dominated by events associated within fracturing and brecciation. The mineralized fractures and brecciated intervals contain complex mineralogies. Multiple minerals, interpreted to be hydrothermal in origin, are present, including magnesite, norsethite, witherite, gorceixite, potassium feldspar, sphalerite, chalcopyrite, and saddle dolomite. CL in minerals within fractures reveals evidence of evolving fluids and multiple fluid-flow events. Although porosity is present within pyrite framboids, between clay sheets, and as dissolution vugs within fractures and allochems, fracture porosity was destroyed by mineralization that filled the fractures and permeated into the matrix. Vitrinite reflectance data from both cores indicate a correlation between thermal maturity and level of hydrothermal alteration, with core A (0.80% VRoVRoVRo [approximately 125°C]) displaying a lower amount of alteration and core C (approximately 1.5% VRoVRoVRo [approximately 210°C]) displaying a higher amount of alteration. The extensive faulting present in the southeastern Anadarko Basin likely facilitated the movement of the hydrothermal fluids through the unit. The presence of hydrothermal minerals not only has implications for the thermal maturity and reservoir quality of the Woodford Shale, but it also contributes to the discussion of whether or not shales behave as open or closed systems.